1. Field of the Invention
This invention relates generally to heat transfer apparatus, and more specifically, to a rotating heated cylinder for producing or processing materials or work pieces. Such cylinders may be used in a number of industries including the pulp and paper industry, various metal rolling industries, the food processing industry, the plastics industry, copy machines, laminating machines, and many other applications. The invention is of particular interest in the pulp and paper industry as a dryer and the metal rolling industry as a roller.
2. Related Art
Multicylinder drying systems currently used in the pulp and paper industry are composed of a series of cylinder dryers as schematically represented in FIG. 1. Such drying systems may use up to about 70 cylinders, but a typical newsprint or fine paper dryer system may use up to about 55 cylinders. The individual drying cylinders of these systems typically comprise rotating pressure vessels that are heated by pressurized steam. The use of pressurized steam as the heating medium for such dryers, however, has several disadvantages. First, to be even minimally effective, the steam in these cylinders must be heated to a temperature in excess of 350.degree. F. At 350.degree. F., the vapor pressure of steam is approximately 135 p.s.i.a. Thus, these cylinders must be constructed to meet pressure vessel codes and standards, making the manufacture of the cylinders expensive and difficult.
Second, as the steam contained inside the cylinders condenses, varying depths of condensate form on the cylinders' inner walls causing them to have a nonuniformly heated outer surface. Similarly, condensate and excess working fluid pools at the "bottom" of the cylinders as they rotate about their horizontal axes. This also impedes uniform heating. These problems, in turn, result in a nonuniform final product.
Third, the pressurized cylinders are inefficient and dangerous to operate. For example, as described above, varying depths of condensate on the inside of a cylinder cause nonuniform heating of the cylinder, and this results in a nonuniform final product (e.g. the paper to be contact dried in a pulp and paper mill is only partially dried). To correct this problem, additional energy is typically added in an attempt to achieve a uniform final product. This, of course, is inefficient. Likewise, the very necessity of meeting steam pressure vessel codes and standards suggests an element or possibility of danger associated with high pressure steam used in these cylinders.
Another problem with prior art cylinders occurs in the aluminum, copper, steel and other industries where metal sheets are rolled from ingots or other feedstock into sheets (see FIG. 2). In these applications, the inability of prior art cylindrical rollers to maintain a uniform roller temperature during rolling of the metal causes an undesirable variation in sheet thickness. As the ingots or other feedstocks are forced through gradually smaller and smaller roll press openings, the surface areas of the rollers coming in contact with the feedstocks heat up. At the ends of the individual rollers, the heat is more easily dissipated than near the middle of the rollers; therefore the rollers expand more around the middle. The result is inefficient use of materials, poor quality control, and variable strength characteristics in metal sheets having nonuniform thickness (i.e., there is a region in the middle of the final metal sheets where the metal is thinner than the outer sides of the sheets).
Various attempts have been made in prior art cylinders to alleviate some of the problems described above. For example, H. L. Smith, Jr., U.S. Pat. No. 3,228,462, describes a cylinder dryer that uses a fluid heat transfer medium, preferably liquid, which flows in opposite directions through two independent, interested labyrinthine flow channels around the periphery of the dryer cylinder. This working fluid is described preferably to be liquid hydrocarbons which may be heated to temperatures of 500.degree.-800.degree. F. and higher without boiling or decomposing to a significant extent. The patent further states that the heat transfer medium is circulated in liquid form at low pressure, eliminating the disadvantages attending high pressure steam and yet permitting higher surface temperatures to be obtained than are practical in steam heated drum dryers.
The cylinders described in the Smith reference, however, have various problems associated with them. For example, most drying facilities are already equipped with steam generating components. Therefore, to implement the Smith dryers on a large scale in already existing factories would be unduly expensive. Furthermore, manufacturing the internested labyrinthine flow channels disclosed in Smith, to achieve even a substantially uniformly heated cylinder, would be highly exacting, expensive and difficult. This is not to mention the expense and difficulty associated with manufacturing such channels and cylinders so that they do not leak the working fluid to undesired locations.
Hemsath, et al., U.S. Pat. No. 4,693,015, uses a direct firing burner which oxidizes fuel and directs hot combustion gases into the center of a dryer. The gases are then recirculated to nozzle assemblies contained in a plurality of extending boxes positioned around the periphery of the dryer cylinder. This system of direct firing of a flammable gas into each individual cylinder is inefficient, expensive and dangerous to operate. Moreover, most pulp and paper factories are equipped with steam heating components, and it would be expensive to replace them all with direct firing burners. Likewise, direct oxidation of a flammable fuel at up to 70 or more dryer cylinder locations, with the attendant possibility of fuel leaks and explosions, can be highly dangerous.
Schuster, U.S. Pat. No. 4,105,896, describes a double-walled hollow cylinder which is heated by an evaporation and condensation chamber formed between the inner and outer walls of the cylinder. This patent further states that the evaporation and condensation chamber has a larger outside diameter at either end of the cylinder than the outside diameter of the rest of the cylinder in between. The larger outside diameter, together with the inner cylinder wall, defines annular compartments or vapor generators at both ends of the cylinder. These annular compartments have steel wool packing in them to enhance vaporization. Upon heating a liquid working fluid contained in these annular compartments with an electrical slip ring/brush combination, the working fluid vapors travel from the annular compartments into the hollow cylindrical chamber defined by the inner and outer cylinder walls, thereby heating the cylinder surface that contacts a work piece.
Unfortunately, Schuster does not solve the problem of varying depths of condensate on the inner cylinder wall causing nonuniform heating of the working surface of the cylinder. Also, the problem of meeting pressure vessel requirements is only partly overcome to the extent that Schuster describes use of a carbon fluoride working fluid having a lower vapor pressure than other kinds of working fluids.
A heat pipe roller used in laminating and copy machines is described in Sarcia, U.S. Pat. No. 4,091,264, and Jacobson et al., U.S. Pat. No. 3,952,798. The heat pipe roller disclosed by these patents uses an internal, axially positioned, heat source and makes use of a wicking structure that extends radially from the heat source to cover the cylinder's inner surface. Likewise, the heat pipe roller of Sarcia and Jacobson contains a working fluid which is partially absorbed into the wicking structure and brought towards the heat source by capillary action, gravity and a paddle wheel-like action resulting from rotating the roller having radially extending wicking components inside.
The foregoing prior art rollers make no attempt to solve the need for costly and difficult pressure vessel construction. Also, such rollers are not suitable for high speed rotation necessary in many roller and cylinder dryer applications. This is because the working fluid of these rollers will be forced out away from the axial heat source as the roller rotates at higher and higher revolutions per minute (rpm's), and thus the working fluid will not be adequately vaporized. Such rollers are therefore limited to slow rotating applications. Also, the references describing these rollers show no awareness of the problems inherent in vaporizing a working fluid inside the roller itself (i.e., varying levels of condensate causing nonuniform heating, and the adverse effects on temperature uniformity of working fluid pooling at the "bottom" of the roller).
Heat pipes per se are well known. Generally, a heat pipe comprises a sealed tube containing a working fluid and a capillary structure. In choosing a suitable working fluid, one skilled in the art will consider the physical properties of the fluid and the desired characteristics of the heat transfer cylinder. "[T]he choice of a working fluid is dependent on physical properties of the fluid and compatibility of the fluid with the wicking structure. Among properties which will be considered by one skilled in the art are: vapor pressure, thermal conductivity, viscosity, and density of vapor and liquid" (see Sarcia, U.S. Pat. No. 4,091,264 citing Articles and U.S. Patents).
The capillary structure in a heat pipe may be made of any suitable material providing capillary attraction to a particular working fluid. For example, grooves etched into the heat pipe, wire lattices, and wicking material have all been used as capillary structures in heat pipes. Energy transfer within a heat pipe is basically accomplished in a cycle. To start the cycle, heat is applied to one end of the pipe (the evaporator part), thereby raising the temperature of the working fluid inside the pipe above its vaporization temperature. As the vapor leaves the evaporator portion of the heat pipe, it fills the rest of the pipe where the temperature is slightly lower than the evaporator part. This causes the vapor, now evenly distributed throughout the heat pipe to condense, thereby releasing additional thermal energy. To complete the cycle, the condensate is drawn back towards the evaporator through the above described capillary structure within the pipe.